Vacuum-operated sewage systems have been developed for vacuum-transportation and collection of sewage discharged from houses and the like. A conventional vacuum-operated sewage system is comprised of a vacuum valve unit including a sewage tank for accumulation of sewage discharged from houses and the like, a water tank located in the vacuum station for collection of the sewage, and a vacuum sewage pipe connecting the vacuum valve unit and the water tank. A vacuum pump is installed in the vacuum station and evacuates the inside of the vacuum sewage pipe.
The vacuum valve unit includes a sewage suction pipe for sucking sewage accumulated under atmospheric pressure in the sewage tank, and a vacuum valve for communication and shut-off between the sewage suction pipe and the vacuum sewage pipe. The opening and closure of the vacuum valve is controlled by making the vacuum sewage pipe vacuous. While the vacuum valve is open, communication between the vacuum sewage pipe and the sewage suction pipe is established to allow the suction of the sewage accumulated in the sewage tank into the evacuated vacuum sewage pipe.
The vacuum sewage pipe is designed either to suck air after the suction of sewage or to suck air and sewage together. The sucked air flows faster in the vacuum sewage pipe than the sewage, which creates a two-phase flow of sewage and air in the vacuum sewage pipe. The two-phase air/liquid flow travels at a high speed and transports the sewage through the vacuum sewage pipe.
While the non-vacuum sewage pipe should be laid inclined in one direction and depend on natural downflow of sewage, the vacuum sewage pipe is usually buried in a shallow ground closer to the ground surface, in a serrated plumbing pattern comprising alternate repetition of downward slopes and upward slopes (lift parts) with a height difference of about 30 cm. Where the course of the vacuum sewage pipe between the vacuum valve unit and the vacuum station is interrupted by obstructions such as a river and other subterranean pipes, the vacuum sewage pipe is arranged to make a detour over or under the obstructions.
In the vacuum sewage pipe of this plumbing pattern, the air travels at a high speed and flows ahead of the sewage in the presence of the two-phase air/liquid flow. The sewage left behind the air remains stagnant at the bottom of a lift part to form a water-seal which seals the sewage pipe. The water-seal sewage at the bottom of the lift part passes the lift part by forming a two-phase air/liquid flow together with another flow of air sucked from the upstream side of the water-seal. The sewage which has passed through the lift part is then trapped at the bottom of the next lift part to form another water-seal. Thus, in the vacuum sewage pipe, sewage is transported beyond the lift parts to the water tank in the vacuum station, with repeating the formations of the two-phase air/liquid flow and the water-seal. The sewage collected in the water tank is then sent pressurised to a sewage treatment plant or the like by means of a pressure pump.
The vacuum-operated sewage system can be classified into a separate air/liquid suction method of sucking the sewage from the sewage tank into the vacuum sewage pipe and sucking the air thereafter (see Japanese Patent Application Laid-open No. 43527/1991 (JP-A-3-43527)), or a simultaneous air/liquid suction method of sucking the sewage and air at the same time (see Japanese Patent Application Laid-open No. 33380/1993 (JP-A-5-33380)). The simultaneous air/liquid suction method also includes the simultaneous-separate air/liquid suction method which independently sucks supplementary air, following the simultaneous air/sewage suction step, so as to compensate for the amount of sucked air.
In any of these methods, sewage and air are sucked into the vacuum sewage pipe normally at a ratio of about 1:3. For example, 40 litres of sewage basically requires 120 litres of air. The sewage/air ratio can be judiciously adjusted for every vacuum valve unit where the sewage and air are sucked off, depending on such conditions as the degree of vacuum obtained in the sewage tank and the plumbing pattern of the vacuum sewage pipe.
The separate air/liquid suction method comprises alternate steps of sucking sewage from a sewage suction pipe and then sucking air from the same sewage suction pipe, whereby a flow of the sucked air transports the sewage efficiently. The amount of the air suction can be controlled by adjusting the air suction time.
According to the simultaneous air/liquid suction method, the vacuum valve unit further includes, besides a sewage suction pipe for sucking sewage, an air suction pipe having a smaller diameter than the sewage suction pipe and disposed downstream of the vacuum valve. Air is sucked from the air suction pipe, while the sewage is sucked from the sewage suction pipe.
In these conventional methods, the ratio of air and sewage to be sucked into the vacuum sewage pipe is regulated at a relatively stable level. In practice, however, the operation of the vacuum-operated sewage system is affected by various causes including the case where the amount of sewage flowing into the vacuum valve unit is not constant throughout the day, the degree of vacuum in the vacuum sewage pipe varies due to the suction of sewage in the neighboring vacuum valve unit, or a two-phase air/liquid flow is not formed in the lift part of the large-diameter vacuum sewage pipe where air flows by itself. These conditions result in air shortage in creating a two-phase air/liquid flow which allows sewage to overflow the lift part, even if air is duly sucked in an amount preset for the clearance of a water-seal. In the end, sewage forms a water-block which completely blocks the lift part. Since the vacuum sewage pipe includes a number of lift parts, the water block may occur suddenly. Once the water-block stops up a lift part completely, the vacuum-operated sewage system is less likely to ensure stable transportation of sewage.
Some causes of the air shortage can be mentioned here. The simultaneous air/liquid suction method employs an air suction pipe of relatively small diameter located downstream of the vacuum valve. According to this structure, the air suction time is limited to the period when the vacuum valve is open. Besides, considering the amount of air intake depends on the diameter of the air suction pipe, the small-diameter suction pipe cannot supply a sufficient amount of air into the vacuum sewage pipe.
On the other hand, the separate air/liquid separation method controls the air suction time and amount into the vacuum sewage pipe by providing a controller or timer on the vacuum valve to control the time of opening the vacuum valve which effects communication and shut-off between the vacuum sewage pipe and the sewage suction pipe. Despite judicious control of the valve opening time, the ratio of air and sewage cannot be maintained in some cases. For example, if the degree of vacuum drops extremely within the vacuum sewage pipe, the vacuum valve, which opens and closes in accordance with the vacuum within the vacuum sewage pipe, may fail to operate properly. As a result, the air intake decreases relative to the sewage intake.
When the vacuum in the vacuum sewage pipe is at an extremely low degree, air needs to be sucked in an increased amount while the vacuum valve is open. By way of example, the separate air/liquid suction method additionally adopts a simultaneous air/liquid suction method of sucking sewage and air together to complement air into the vacuum sewage pipe. However, as described above, according to the simultaneous air/liquid suction method, air is sucked in a limited amount through the relatively small-diameter air suction pipe only when the vacuum valve is open. Thus, the air shortage problem in the vacuum sewage pipe cannot be solved simply by adopting the simultaneous air/liquid suction method.
Alternatively, in the separate air/liquid suction method, the vacuum valve unit can be designed to detect the completion of the sewage suction and the start of the air suction in the sewage tank, thereby to close the vacuum valve after a predetermined period of the air suction. This solution still fails to ensure sufficient air supply into the vacuum sewage pipe, in case the vacuum in the vacuum sewage pipe is at an extremely low degree.
Japanese Patent Application Laid-open No. 319662/1996 (JP-A-8-319662) discloses a vacuum-operated sewage system comprising a plurality of air intake ducts each connected to the upstream side neighboring the lift part in the vacuum sewage pipe, the top end of each air intake duct being located at the ground surface and provided with an air inlet valve. The air inlet valve is allowed to open when a water-block formed at the bottommost portion of the lift part causes the drop of the degree of vacuum in the vacuum sewage pipe on the upstream side thereof. Air on the ground is introduced through the open air inlet valve into the vacuum sewage pipe and eventually clears the water-block formed therein.
The air inlet valve provided at the top end of the air intake duct has a simple structure comprising a cylindrical housing which covers the top end of the air intake duct, and a valve member disposed opposite to the top end surface of the air intake duct and held inside the housing by a compression spring equipped therein. When the vacuum is created in the air intake duct which communicates with the vacuum sewage pipe, the valve member is sucked against the spring stress of the compression spring to close the top surface of the air intake duct. On the other hand, when the degree of vacuum drops in the air intake duct, the valve member opens the top surface of the air intake duct under the spring stress of the compression spring.
Nevertheless, the structure of the air inlet valve is too simple to operate the opening and closure thereof sensitively in response to the drop of the vacuum within the vacuum sewage pipe. In the end, the air inlet valve may fail to permit a quick and sufficient air supply into the vacuum sewage pipe.
As mentioned above, this sewage system provides a plurality of air inlet valves each at the top end of a plurality of air intake ducts connected to the vacuum sewage pipe. In case the pressure inside the vacuum sewage pipe is released to the atmosphere for such troubles as breakage of the vacuum sewage pipe, all of the air inlet valves are allowed to open. Following the recovery from the trouble (e.g. by repairing the vacuum sewage pipe), the inside pressure of the vacuum sewage pipe needs to be brought back to the normal vacuum state. However, it is difficult to evacuate the entire range of the vacuum sewage pipe, with all air inlet valves remaining open to the atmosphere.
This problem can be solved by providing a switch valve to every air inlet valve and operating the switch valve to the closed position. However, considering the extensive distribution of a number of switch valves, it is laborious to close all of them.